In recent years, composite metal oxides have been developed and used in many applications such as catalysts, catalyst supports, adsorbents, electrodes, magnetic materials and electronic components, because of their unique properties.
In the field of an exhaust gas purifying catalyst, using a composite metal oxide as the catalyst support is attracting attention.
Conventionally, in the field of an exhaust gas purifying catalyst, in order to obtain a large surface area, alumina (Al2O3) has been generally used. However, in order to accelerate purification of an exhaust gas by using chemical properties of the support, recently it has been proposed to use a composite metal oxide containing various metal oxides such as alumina (Al2O3) ceria (CeO2), zirconia (ZrO2), titania (TiO2) and silica (SiO2), as a porous metal oxide support.
As the composite metal oxide support, a composite metal oxide of ceria and zirconia is generally known. A ceria-zirconia composite metal oxide can have significantly improved heat resistance compared with ceria alone, while providing the OSC (Oxygen Storage Capacity) of ceria, i.e., the capacity of storing oxygen when the oxygen concentration in an exhaust gas is high and releasing oxygen when the oxygen concentration in the exhaust gas is low.
Ceria-zirconia composite metal oxide is generally produced by a coprecipitation method of preparing an aqueous salt solution containing a cerium salt and a zirconium salt, making the aqueous salt solution basic pH to precipitate a ceria-zirconia composite metal oxide precursor, particularly a ceria-zirconia composite hydroxide, and then drying and firing the precursor.
Japanese Unexamined Patent Publication No. 5-49864 proposes to use a catalyst comprising metallosilicate, as an exhaust gas purifying catalyst other than a noble metal-supported alumina. The metallosilicate used here is a compound having a structure where at least a part of aluminium in zeolite is replaced by other elements. It is described that the element replacing aluminium in zeolite is preferably an element similar to aluminium in the ion radius or chemical properties, and iron, gallium, zinc and lanthanum are described as specific examples thereof.
In Japanese Unexamined Patent Publication No. 5-49864 above, an alkali metal-type metallosilicate is produced by the hydrothermal processing of a silica source and an alkali metal source, and then the alkali metal of the alkali metal-type metallosilicate is ion-exchanged with a desired metal, whereby the above-described metallosilicate is produced.
Japanese Unexamined Patent Publication No. 2001-314763 discloses a support for a NOx storage-reduction type catalyst, the support containing a titania-zirconia composite metal oxide and an element selected from the group consisting of lanthanum, neodymium and praseodymium.
In Japanese Unexamined Patent Publication No. 2001-314763 above, the titania-zirconia composite metal oxide is produced by a general coprecipitation method of mixing zirconium chloride and titanium tetrachloride in hydrochloric acid, and adding dropwise ammonia thereto to perform coprecipitation. Furthermore, this titania-zirconia composite metal oxide is dipped in a salt solution, such as lanthanum nitrate solution, and then dried and fired, whereby a support for NOx storage-reduction-type catalyst is obtained.
Japanese Unexamined Patent Publication No. 8-266865 discloses an exhaust gas purifying catalyst obtained by loading a platinum group element on a catalyst support layer such as silica, zeolite, silica-alumina and titania-alumina, wherein a composite metal oxide of vanadium and lanthanum or the like is further loaded on the catalyst support layer.
Incidentally, Japanese Unexamined Patent Publication No. 2002-282692 discloses a technique of suppressing sintering of rhodium by allowing a lanthanoid element present in zirconia to exert an anchoring effect, i.e., an effect of suppressing movement of rhodium on the zirconia surface.
Regarding the production process of a composite metal oxide, a method of mixing a plurality of kinds of powdery metal oxides and firing the mixture at a very high temperature; a method of dipping a metal oxide particle in a metal salt solution and then drying and firing the particle at a high temperature; and a so-called coprecipitation method are known.
Among these production processes, the coprecipitation method is preferred, for example, in that a composite metal oxide having a uniform composition can be obtained and that a composite metal oxide can be obtained by firing at a relatively low temperature.
However, in the coprecipitation method, it is difficult to obtain a uniform composite metal oxide of a basic metal oxide such as an alkaline earth metal oxide and rare earth oxide, and an acidic metal oxide such as silica and titanium. This is because the ion of a basic metal, for example lanthanum ion, forms a precipitate only at a relatively large basic pH, whereas the ion of acidic metal, for example silicon ion, forms a precipitate in a wide pH range excluding strongly acidic and strongly alkaline regions.
More specifically, in the case of performing the coprecipitation method by using a basic metal salt, such as lanthanum nitrate and an acidic metal salt, such as sodium silicate, the pHs at which these salts form precipitates significantly differ from each other, and the precipitates are formed at different timings. The precipitates containing respective metals form separate aggregates, and therefore a uniform precipitate containing the both metals can be hardly obtained.
Regarding the production of a composite metal oxide, use of a microemulsion method may be considered. In the microemulsion method, water is dispersed in a hydrophobic solvent together with an appropriate surfactant to obtain a liquid dispersion wherein fine water droplets are dispersed in the hydrophobic solvent. Thereafter, a water-soluble metal salt is added to the liquid dispersion, and then a metal hydroxide is precipitated in the fine water droplet dispersed in the hydrophobic solvent.
According to the microemulsion method, a composite metal oxide having a fine secondary particle diameter corresponding to the amount of the metal element contained in the fine water droplet can be obtained.
However, in this microemulsion method, a large amount of an organic solvent must be used in order to form fine water droplets, and this method is disadvantageous in terms of the cost of recovery and treatment of the organic solvent.
Accordingly, the present invention provides a production process of a composite metal oxide, the process enabling a composite metal oxide containing a plurality of kinds of metals having different properties to be easily obtained at a low cost.